Myotonic muscular dystrophy (DM) is the most common form of muscular dystrophy in adults and is second to Duchenne's muscular dystrophy in incidence of new cases of inherited-muscular dystrophies. DM is inherited as an autosomal dominant defect with incomplete penetrance and variable expressivity, and severity of the disease ranges from obligate heterozygotes with no detectable symptoms to profoundly disabled individuals. Although usually considered a skeletal muscle disease, DM, like Duchenne/Becker dystrophy, has a high incidence of serious cardiac involvement. The most specific cardiac signs are conduction abnormalities, including arrhythmias that may result in sudden death. Some patients benefit from pacemaker implantation. In addition to conduction abnormalities, dilated cardiomyopathy and mitral valve prolapse are also observed. Other affected organs are the lens, gastrointestinal and genitourinary systems. Since DM, like Fragile X syndrome, displays the phenomenon known as anticipation (increasing disease severity in subsequent generations) we screened cosmid libraries constructed from two yeast artificial chromosomes spanning the DM region with a set of oligonucleotide probes comprised of tandem trinucleotide repeats, to search for potentially unstable repeat sequences. We isolated a DNA probe, pMYD 1, containing 11 tandem GCT repeats from two overlapping cosmid clones spanning the DM region. The pMYD1 probe recognizes a progressive enlargement of a single Nco 1 restriction fragment in congenitally affected DM patients born to affected mothers. Using pMYD1 as a probe, a 3.2-kb cDNA clone was isolated from a fetal brain cDNA library. The cDNA probe hybridizes to a 3.2-kb transcript in myocardium, Purkinje fiber, skeletal muscle, and brain. Nucleotide sequence analysis of the cDNA identifies a single open reading frame coding for a unique protein with similarity to numerous protein kinases having the tandem GCT repeat sequences in the 3' untranslated region. Synthetic peptide antigens were synthesized from the deduced sequence, and antibodies specific for the DM kinase have been prepared and characterized. A eukaryotic expression system has been developed, and sense and antisense RNA have been transcribed and used for in vitro translation and nuclease protection respectively. The long-term goal of the proposed research is to examine the role of the protein kinase in the pathophysiology of the conduction abnormalities found in patients with DM. In order to examine these questions it will be necessary to know the tissue- and cell- specific expression of DM mRNA within myocardial tissues, the subcellular localization of the DM protein, the developmental pattern of DM expression in myocardium, and the functional characteristics of the protein. The proposed studies are designed to generate the essential biological reagents and to examine DM gene expression and protein function in myocardium and the cardiac conduction system.